Doped polycrystalline Si is often used to fill contact holes or trench capacitors in dynamic random access memories (DRAMs). The polycrystalline Si can be patterned by chemical mechanical polishing (CMP), stopping on an underlying layer of patterned Si.sub.3 N.sub.4, SiO.sub.2, or other suitable materials.
CMP may be accomplished by mounting a wafer coated with polycrystalline silicon onto a carrier and putting it on a polishing machine. An appropriate polishing pad material is driven across the wafer surface. Then, a colloidal silica slurry of an alkaline solution and SiO.sub.2 particles is contacted with the wafer. OH ions in the slurry react with Si atoms, catalyzing the corrosive action of water which results in the cleavage of Si back bonds. This is the mechanical part of the process. After polishing, the wafers are treated with certain chemical agents to remove the polishing slurry. Finally, the wafers are cleaned.
A polishing agent commonly used in the industry for silicon polishing is silica sol. U.S. Pat. No. 4,462,188 discloses the inclusion of up to 5% of an amine in the sol and the addition of a quaternary ammonium salt or base to the amine silica-sol combination. Also, it teaches the use of a silica sol that contains up to 5% of an amine and up to 5% of a quaternary ammonium salt or base.
The sol used in the polishing practices taught in the '188 patent is received with about 30 to 60% SiO.sub.2. The required amine is added. The sol is diluted to about 3% SiO.sub.2 (10 to 20 times) with water. The pH of the diluted sol is adjusted to a range of 10.5 to 11 with alkali metal or ammonium hydroxide. Potassium hydroxide is described as a preferred alkaline solution, and NaOH and NH.sub.4 OH are described as less desirable. Specifically, with these solutions, a higher pH is required to improve stock removal, as shown in U.S. Pat. No. 4,549,374.
The sol is used to polish the device soon after dilution and pH adjustment, before the pH changes significantly. The pH has a tendency to decrease over time and as polishing proceeds. Both of these characteristics require that additional KOH be added to establish or sustain the desired removal rate of the layer.
It has been known in the art to polish silicon metal wafers by using a stable silica sol-amine combination that contains sufficient amine to provide a high pH on dilution. Also, the addition of alkali metal before and during the polishing process has been eliminated such that the only alkali metal present is that which is required during sol preparation.
Conventional slurries do not have reduced solids content because, generally, it is desirable to use as high a solids content as possible to maximize the polish rate. The prior art specifies certain solids content for slurries.
U.S. Pat. No. 3,715,842 describes a silica slurry, used to polish Si or Ge, that is stabilized with a cellulose derivative. The slurry is used in an alkaline solution. Nevertheless, the '842 patent does not recognize the problem of localized oxide erosion because the disclosed process was developed for Si wafer polishing. The proposed solids content in the '842 patent (1 to 50%) would cause considerable insulator erosion if used with the process of the present invention.
U.S. Pat. No. 3,552,071 describes polishing of Si using alkaline earth metal hydroxides, such as calcium hydroxide and strontium hydroxide, in the slurry. This slurry would not provide adequate selectivity to the underlying SiO.sub.2 layer used in the present invention. The silica-based slurries are preferred because they provide high selectivity to the underlying SiO.sub.2.
U.S. Pat. No. 3,552,071 describes polishing of Si using silica sols or gels. Although a silica slurry is used, it is not in an alkaline solution, which is necessary to achieve a high polish rate of Si. In addition, the solids content is relatively high (2 to 50%) and would cause localized erosion of the insulator.
U.S. Pat. No. 4,892,612 describes a silica slurry used to polish Si or Ge having amine additives to enhance the polish rate. Like in the present invention, a silica slurry is used in an alkaline solution. A low solids content is also specified, i.e. "less than 3% SiO.sub.2." Nevertheless, the pH is relatively high (10.5 to 11) which would result in a higher oxide removal rate. A lower pH is preferred in order to minimize oxide erosion.
For patterning polysilicon, a colloidal silica slurry (sol) in an alkaline solution is typically used. The alkaline solution is a necessary component of the slurry which prevents the silica particles from agglomerating. This agglomeration could cause scratches in the surface to be polished and decreased polishing.
CMP is widely accepted as the preferred process for planarizing dielectrics and patterning tungsten (W) studs or polysilicon studs (particularly for specifications of 0.25 .mu.m and beyond) and other surfaces atop a silicon substrate. Nevertheless, CMP does encounter certain defects. These defects include pattern effects, such as oxide erosion and dishing, which increase with polish time and depend on the localized and adjacent patterns. Erosion has a specific pattern to it which seems related to pad bending or trapping of slurry in topography.
Improving the manufacturability and widening process margins in CMP by understanding and improving systematic degradations in performance such as pattern erosion and underlying oxide thickness variations is desired. These pattern effects depend on the specific choice of polisher parameters, consumables, and their effects.
As shown in FIG. 3A, erosion is the thinning of insulators and conductors in an array. Dishing is the thinning of a large feature (such as a conductor) near the center of the feature (as shown in FIG. 3B).
Patterning of W studs by CMP is in many ways similar to patterning of polysilicon studs by CMP, particularly because of the problems that are encountered. As shown in FIG. 4, oxide erosion during W CMP increases with polish time and is slightly higher at the wafer edges where the film removal is faster. When several arrays of vias are profiled, results, shown in FIG. 5, indicate that the edge array shows higher erosion compared to the center array but erosion in both cases is lower than an isolated array with the same via pattern density.
In the case of soft polishing pads, as shown in FIGS. 6, 7, and 8, the erosion pattern is rather specific, but similar to that reported earlier. This type of erosion tends to be preferentially higher towards the edge of a pattern array. FIGS. 6 and 7 show the erosion to be symmetric along the center of the array for isolated arrays or the center array, respectively. For an edge array, as shown in FIG. 8, erosion is higher near the periphery that is distant from the topography. These subtle erosion features may be caused by localized variations of pressure being applied on the wafer surface. First, pad bending is expected to be higher near the edge of an array as the pad encounters topography. Once over topography, the variation in the extent of subsequent pad bending is minimal. The peripheral erosion in an array may also be the result of the increased contribution to film removal from higher localized chemical or mechanical abrasion from the slurry trapped in patterns.
FIG. 1 shows the semiconductor device polished with conventional slurries and depicts an array 14 consisting of a polish stop layer 4 (such as SiO.sub.2 or Si.sub.3 N.sub.4) over a silicon wafer 2. Some examples are the formation of polysilicon studs in boro-polysilicate glass (BPSG) or polysilicon-filled trench capacitors in a Si.sub.3 N.sub.4 pad nitride. These slurries cause defects such as edge erosion 8. The polysilicon studs 10 are also affected by the erosion 8. This erosion 8 creates troughs 6 at the edge of the array that can trap material during subsequent CMP steps, and hence cause leakage.
The deficiencies of the conventional processes and slurries show that a need still exists for a polishing process and slurry that will minimize edge erosion and corresponding defects. To overcome the shortcomings of the conventional processes and slurries, a new process and slurry are provided.